Satan

The rogue planet Satan bypassing the blue giant Beta Crucis in a hyperbolic orbit becomes a base for the industrially valuable transformation of elements into heavier isotopes when its cryosphere becomes atmosphere and hydrosphere, coolants for heat waste.

Falkayn spells out why Satan alone is ideal for producing heavier isotopes. This cannot be done:

on inhabited planets because heat and radioactive waste would make them uninhabitable;
on uninhabited planets because heat waste added to solar radiation would vaporize the rivers needed for coolants;
on uninhabited planets with orbiting albedo-raising dust clouds because these would trap home-grown heat;
in newly formed systems because factories would be bombarded by meteors and asteroids;
on airless planets because the necessary heat exchangers are expensive and put engineering limits on the size of a plant;
on Jovoid planets because free hydrogen diffuses through materials and interferes with nuclear reactions;
on ordinary rogue planets because temperatures near absolute zero affect the properties of matter and because liquid water and gaseous atmosphere are necessary coolants.
-copied from here.